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Enhancing the reactivity of petroleum coke in CO2 via co-processing with selected carbonaceous materials Ashak Mahmud PARVEZ, Yu Hong, Edward Henry Lester, and Tao Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02000 • Publication Date (Web): 02 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017
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Energy & Fuels
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Enhancing the reactivity of petroleum coke in CO2 via co-
2
processing with selected carbonaceous materials Ashak Mahmud Parvez a, Yu Hong a, Edward Lester b, Tao Wu a,*
3 4 5 6 7 8 9
a
Ningbo Municipal Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, Ningbo 315100, China
b
Department of Chemical and Environmental Engineering, The University of Nottingham, Nottingham NG7 2RD, U.K.
Abstract
10
This work presents a novel approach to enhance the gasification reactivity of highly
11
unreactive petroleum coke by purposely promote its synergistic interactions with selected
12
carbonaceous materials. To achieve this, an Australian coal and gum wood were chosen for
13
co-processing with petroleum coke. It was found that the addition of gum wood resulted in
14
more significant enhancement in the gasification reactivity of petroleum coke, which was
15
attributed to the combined influence of the catalytic effect of alkali and alkaline earth metals
16
(AAEM) in gum wood and the unique features of bio-char, such as high surface area, more
17
active sites, low crystalline index etc. It confirmed that interactions between an unreactive
18
fuel, such as petroleum coke, and selected carbonaceous materials could be capitalized on to
19
enhance overall reactivity of the blends. This approach helps improve the conversion
20
efficiency of unreactive fuels such as petroleum coke and therefore promotes their large-scale
21
utilization.
22
Key words: Petroleum Coke; Gasification; CO2 Gasification Reactivity; Interactions;
23
Synergy.
24
1 Introduction
25
With the increasing consumption of crude oil, the production of petroleum coke increases as a
26
by-product from oil refining industry 1-3. Due to its inert nature, it is a big challenge to utilize
27
petroleum coke in large scale and in a sustainable manner. In the past decades, considerable
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research has been carried out on the utilization of petroleum coke via combustion, pollutants
29
emission, and agglomeration
30
biomasses has also been investigated extensively
31
coke en route to energy and chemicals production is considered as one of the attractive
32
methods for its utilization in large scale 3, 7, 11, 12. It is projected that the amount of petroleum
33
coke to be utilized will rise in the near future as more petroleum coke processing units are
34
being added to meet the increasing fuel demand
35
coal or biomass is a promising option for its large-scale utilization in terms of capital
36
investment, greenhouse gas emission and energy security 1, 14, 15.
37
CO2 can be used as a gasifying agent, the use of which can result in less net CO2 emission and
38
lower steam consumption
39
adjust syngas composition for various downstream applications, and contributes to greater
40
environmental and economical benefits for the entire process
41
been made to investigate biomass gasification in CO2 atmosphere
22-24
42
16, 23, 24
. The effects of operating
43
parameters, such as temperature, pressure and heating rate, on the CO2 gasification of
44
petroleum coke, coal and biomass had also been reported
45
very difficult to gasify due to its low reactivity. This is particularly a technical problem in
46
CO2 gasification since CO2 is a much weaker oxidizing agent. To date, not much research on
47
CO2 gasification of petroleum coke mixed with biomass/coal has been reported
48
of the huge potential of CO2 as a gasifying agent, not much work has been conducted to study
49
the synergistic effects under CO2 gasification and how reactivity of a poor fuel could be
50
enhanced.
51
In this study, the objective was to enhance the gasification reactivity of a poor fuel, i.e.,
52
petroleum coke, via the promoting its synergistic interactions with other carbonaceous
4-7
. The co-firing of petroleum coke with coal and different 8-10
. Normally, gasification of petroleum
2, 3, 13
. Gasification of petroleum coke with
16-19
. More importantly, the use of CO2 offers the flexibility to
, gasification reactivity
12, 18, 25
, and characteristics
19-21
. Much effort has therefore , such as kinetic study
12, 26, 27
2, 11, 12
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13, 15
. In spite
2
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materials, such as coal and biomass. CO2 gasification was carried out using a
54
Thermogravimetric Analyser (TGA) under isothermal conditions. Moreover, co-gasification
55
of a suite of samples with air/CO2 was also conducted to demonstrate how to select proper
56
carbonaceous sample to achieve the maximum reactivity of petroleum coke under CO2
57
gasification conditions.
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2 Materials and method
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2.1 Materials
60
The raw materials used in this research were petroleum coke (SINOPEC, China), Australian
61
coal (NSW, Australia) and gum wood (Huzhou, China). All the samples were prepared
62
following British Standard for sample preparation to ensure representativeness of the samples
63
28
64
for future use. In order to show significant interactions between individual components, a
65
blending ratio of 1:1 were adopted in this study. Ash samples of these materials were also
66
prepared based on British Standard 29.
67
2.2 Sample characterization
68
Proximate analysis was carried out using a TGA (NETZSCH STA 339 F3) following the
69
procedures adopted elsewhere
70
contents were measured using a PE 2400 Series II CHNS/O Elemental Analyser (Perkin
71
Elmer, USA). In each test, approximately 3 mg (accuracy up to 0.01 mg) of dried fine powder
72
of the sample was used, which was manually grinded further prior to testing to eliminate the
73
influence of particle size on diffusion. Each test was repeated at least three times to minimize
74
the experimental error. The C, H, N and S contents of samples were measured directly, while
75
O content was calculated by difference. Calorific values of individual samples were
76
determined using a calorimeter (IKA C 200, Germany), which was calibrated prior to testing
77
using benzoic acid to achieve a relative standard deviation less than 0.2%. As for ash
. Approximately 1.0 kg of each sample was grinded into a size range of smaller than 106 µm
30
. Carbon (C), hydrogen (H), nitrogen (N) and sulphur (S)
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composition, each sample was analysed three times using an X-Ray Diffraction (XRD)
79
(Bruker D8 Advance, Germany) to minimize experimental error to be within +/- 1.5%. The
80
stacking height (LC) and interplanar spacing (d002) of char samples were calculated following
81
the formula described elsewhere 11, 31. The surface properties such as BET (Brunauer-Emmett-
82
Teller) surface area and pore volume of each sample were measured by accelerated surface
83
area porosimetry instrument (ASAP 2020, Micromeritics Instrument, Inc., USA) with N2
84
adsorption.
85
CO2 chemisorption test of char samples was conducted in a TGA by adopting similar
86
experimental procedure reported elsewhere 32, 33. At the beginning of each test, approximately
87
15 mg of the sample was heated to 850 °C (20 K/min) in pure N2 (99.99%). After 30 min
88
outgassing process at 850 °C, the sample was cooled down to 300 °C and then stabilized for
89
another 30 min. CO2 chemisorption was started when N2 was switched to CO2 and was held
90
for 30 min. The change in weight was continuously monitored. Finally, the CO2 was switched
91
back to N2, followed by outgassing for 30 min to remove weakly chemisorbed molecules from
92
the surface. The flow rates of both N2 and CO2 in this process were set as 120 mL/min. From
93
the test, two sets of chemisorption data were obtained for each sample. The first one gave the
94
total chemisorbed CO2 volume (strong and weak chemisorption), denoted by Ctot, while the
95
second one was the strong chemisorption of CO2 (Cstr) at 300 oC, which was still absorbed on
96
the char surface even after the experiment was finished. The weak chemisorption of CO2
97
(Cwea) was calculated through the desorbed volume from the char surface during the
98
outgassing stage.
99
2.3 Combustion and gasification characteristics
100
Combustion and gasification experiments were carried out using the TGA. Prior to each test,
101
the sample was further grounded manually in a mortar to a size even smaller than 106
102
microns. During non-isothermal test, the sample prepared was heated at a heating rate of 20 ACS Paragon Plus Environment
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K/min from the ambient temperature to 1200 °C under the presence of air or CO2 (40
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ml/min). In isothermal gasification test, the sample was heated to 1200 °C in N2 (30 ml/min),
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switched to CO2 (40 ml/min) and kept isothermal until no evident of mass loss was detected.
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Each test was repeated three times to minimize experimental error.
107
2.4 Reactivity indexes
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In this study, several parameters, such as ignition temperature (Tig), peak temperature (PT, Tp)
109
and burnout temperature (BT, Tb), were adopted as indicators to assess the combustion
110
behaviours of the blends
111
substance continues to burn without the need for external heat supply. Peak temperature is the
112
temperature at which the combustion rate reaches the maximum, while the temperature under
113
which combustion rate falls below 1 wt%/min is defined as the burnout temperature. Ignition
114
index (Di), as expressed in Eq. (1), was also used to measure ignition performance of fuels 34.
115
Di =
116
where, Rmax (%/min) and tmax represent the maximum combustion rate and corresponding time
117
(min), respectively, while tig stands for ignition time.
118
Combustion index (Sc) is determined by 36:
119
Sc =
120
where, Rmax, Rmean, Tig and Tb are the maximum mass loss rate, average mass loss rate,
121
ignition temperature and burnout temperature, respectively. Normally, greater values of Di
122
and Sc suggest better ignition and combustion performance, respectively.
123
Based on the TGA data, carbon conversion (Xc) and gasification reaction rate (RG) of the
124
studied samples can be calculated using Eq. (3) and Eq. (4), respectively 11.
34, 35
. Ignition temperature is the minimum temperature at which a
Rmax
(1)
tmax ⋅tig
Rmax ⋅Rmean
(2)
Tig2 ⋅Tb
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XC =
m0 − m m0 − mash
(3)
126
RG =
dX C dt
(4)
127
where m is the mass at a particular time, mo is the initial mass and mash is ash content, t
128
represents the time of gasification reaction.
129 130
2.5 Interactions indexes
131
Thermal characteristics of petroleum coke, Australian coal, gum wood and their blends were
132
extracted from experimental DTG profiles in air/CO2. The theoretical DTG curves of the
133
blends were calculated using Eq. (5) based on the mass loss rates of each sample assuming
134
additive property applies. Any deviation between experimental and calculated DTG curves
135
was used as indication for the interactions between samples, where the higher the value, the
136
greater the interactions 1, 37, 38.
137
dm dm dm = x SP1 + x SP 2 dt dt SP1 dt SP 2
138
dm dm where are the mass loss rates (%/min) of individual samples while and dt SP1 dt SP 2
139
x SP1 and xSP2 are the corresponding mass fractions in the blends, respectively. The Root
140
Mean Square Interactions Index (RMSII) was used to measure the interactions between
141
components in the blend, which compares the deviation of calculated values with
142
experimental values. Normally, a greater RMSII value indicates a stronger interaction
143
between the samples. The RMSII can be calculated using the Eq. (6) 39.
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(dm / dt ) iEXP − (dm / dt ) iCAL ∑ (dm / dt ) iCAL i =1 N N
2
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RMSII =
145
where ( dm / dt ) EXP and (dm / dt) CAL denote experimental and calculated mass loss rates,
146
respectively. N represents the number of points undertaken.
147
3 Results and discussion
148
3.1 Characteristics of samples
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Results of proximate, ultimate and ash analyses as well as calorific value of the investigated
150
samples are listed in Table 1. It can be seen that gum wood contained the highest amount of
151
volatiles, over twice and eight times as much as those of Australian coal and petroleum coke,
152
respectively, but significantly less amount of fixed carbon (11.8 wt%) and ash (0.1 wt%). In
153
addition, gum wood also had the lowest content of sulphur among the three samples. From
154
Table 1, petroleum coke (36.4 MJ/kg daf) had the highest calorific value, comparable to that
155
of Australian coal (35.5 MJ/kg daf). Hence, co-processing of coal with petroleum coke might
156
not result in considerable decrease in combustion/gasification temperature in utility boilers or
157
gasifiers. In contrast, gum wood (14.9 MJ/kg daf) had the lowest calorific value, which would
158
result in a lower processing temperature if co-processed with coal.
159
Table 1: Characteristics of samples.
(6)
PC
AC
GW
36.4
35.5
14.9
Moisture
0.8
0.7
2.1
Volatile matter
10.9
34.6
86.0
Fixed carbon
87.1
48.2
11.8
Ash
1.2
16.5
0.1
Net calorific value a,b (MJ/kg) Proximate analysis c (wt %)
Ultimate analysis a, b (wt %)
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C
91.1
81.3
47.1
H
3.8
4.9
6.3
Od
2.6
9.7
43.5
N
1.3
1.9
2.1
S
1.2
2.2
1.0
SiO2
36.8
42.6
52.5
SO3
6.1
1.4
1.2
CaO
3.4
4.2
7.6
Na2O
6.4
2.3
2.9
Fe2O3
9.5
7.4
7.3
MgO
2.3
2.2
6.8
Al2O3
33.9
39.1
17.2
K2O
1.6
0.8
4.5
Ash analysis (wt %)
a
Dry basis. bAsh free basis. cAs-received basis. cBy difference.
160
3.2 CO2 gasification reactivity
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Fig. 1 presents mass loss during pyrolysis stage (prior to 1000 ºC) and CO2 gasification stage
162
(isothermal at 1000 ºC) of different samples. In general, gum wood had the highest mass loss
163
rate in both pyrolysis and gasification stages. The reasons behind this fastest reactivity can be
164
attributed to its high volatile and oxygen content as well as thermal degradation properties of
165
the sample’s constituents, i.e., lignin, cellulose, hemi-cellulose
166
fundamental aspect of gasification showed that the concentration of active sites in the char is
167
directly related to the oxygen-containing functional groups of the parent sample
168
was significantly higher in gum wood than in petroleum coke. Furthermore, AAEMs in gum
169
wood might also have positive impacts on gasification reactivity as normally AAEMs are
170
catalysts to both combustion and gasification processes
171
Australian coal and petroleum coke samples showed much lower reactivity, which can be
172
explained by their lower volatile and oxygen content and higher fixed carbon content. For
173
petroleum coke, the highest amount of final residue (about 65 wt%) was found. This could be
35, 37, 40, 41
. Research on
37, 40
, which
42, 43
. Compared with gum wood,
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44-46
174
attributed to its aromatic nature and high ordered carbon fraction
175
that aromatic and graphitic (high ordered) carbon atoms had lower reactivity than aliphatic
176
and the disordered carbon, respectively
177
amount of carbon while the oxygen content was the lowest. It can be seen in Fig. 1 that
178
complete conversion of gum wood was achieved, the conversion of Australian coal was yet
179
not complete, while only about 35 wt% of petroleum coke was converted. Therefore, the
180
testing of petroleum coke was conducted at higher temperatures.
. It is important to note
46
. Moreover, petroleum coke contained the highest
100
1200
PC
Temp 1000
800 60 600 40 400
Temperature (oC)
80
Mass (%)
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20
200
GW 0
0 0
20
40
60
80
100
120
Time (min)
181 182
Figure 1: Thermal behaviours of individual samples upon heating
183
The comparison of CO2 gasification profiles of petroleum coke at 1000 ºC, 1100 ºC and 1200
184
ºC under isothermal conditions is shown in Fig. 2. The results demonstrated that compared
185
with other carbonaceous materials, to raise the CO2-gasification rate to a reasonable level, a
186
much higher operating temperature must be adopted, which is in accordance with the analyses
187
of previous investigation
188
has to be conducted at a temperature higher than 1200 ºC. Thus, in order to compare
189
gasification reactivity of different samples, experiments were performed at 1200 ºC, the
190
results of which are shown in Fig. 3a.
2, 25
. To achieve a high conversion of petroleum coke, gasification
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100
1400 1200 oC
1200 1100 oC 1000 oC
1000
60
800
PC (1000 oC)
40
PC (1100
600
oC)
Temperature (oC)
80
Mass (%)
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400 20 200
PC (1200 oC) 0
0 0
20
40
60
80
100
120
191
Time (min)
192
Figure 2: CO2 gasification profiles of petroleum coke at different temperatures.
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100
1500
PC
(a)
Temp 1200 Temperature (oC)
80
Mass (%)
AC 60
900
40
600
GW 20
300
0
0 0
20
40
60
80
100
120
Time (min)
193 1
(b)
AC-Iso
1200 oC
0.8 Conversion (XC)
GW-Iso
0.6
PC-Iso 0.4
0.2
0 0
194
15
30 Time (min)
45
60
0.8
1200 oC (c)
Gasification rate (RG)
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GW-Iso
0.6
0.4
PC-Iso
0.2
AC-Iso
0 0
0.2
0.4
0.6
0.8
1
Conversion (XC)
195 196
Figure 3: Pyrolysis (a, b) and CO2 gasification (c) profiles of individual samples.
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The thermal profiles at 1200 ºC were comparable to those found in 1000 ºC but high
198
conversion was observed for Australian coal and petroleum coke. The profiles of carbon
199
conversion versus gasification time and gasification rate of the tested samples using
200
isothermal method are presented in Fig. 3b and Fig. 3c, respectively. It can be seen in Fig. 3b
201
that gum wood and Australian coal had 75 wt% and 98 wt% of conversion in 3 min and 9
202
min, respectively, demonstrating their high reactivity in CO2 atmosphere. In contrast, the
203
conversion of petroleum coke experienced an exponential rise over the entire period of
204
testing. This gradual increase in conversion confirmed the sluggish onset of the Boudourd
205
reaction. This was due to the low reactivity of petroleum coke as mentioned earlier.
206
The gasification rate of gum wood increased initially and then decreased after reaching a peak
207
value as seen in Fig. 3c. It remained a constant thereafter. The conversion corresponding to
208
the maximal gasification rate was around 0.25. The presence of maximum gasification rate
209
could be explained by the evolution of pore structure of the solid fuel, leading to a higher
210
surface area and eventually a higher number of active carbon sites per unit weight
211
development of porosity in the solid fuel is closely associated with the formation of pore
212
clusters and the continuously consumption of carbon materials. During the gasification
213
process, the increase in accessible porosity takes place on the available solid sites at the
214
boundary of a pore cluster. Consequently, the rise of gasification rate with the conversion was
215
observed due to the enlargement of accessible surface area. Thereafter, the pores collapsed
216
and resulted in a reduction of reaction surface area, which subsequently led to the drop of
217
gasification rate. For Australian coal, a comparable pattern was obtained which could be
218
explained by the evolution of pore structure. However, the maximum gasification rate found
219
to be lower than that of gum wood. This was due to the fact that char derived from coal is
220
normally less reactive than that derived from biomass. It was also found that gasification rate
221
of petroleum coke gradually decreased during testing and the rate was the lowest among all
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. The
12
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222
samples. These properties are closely associated with graphitization phenomenon
. In
223
comparison with chars derived from both coal and biomass, petroleum coke is generally a
224
part-graphitized carbon material with higher crystallinity and higher degree of order 2. It was
225
reported that the degree of graphitization in petroleum coke was greater than that of chars
226
derived from coal at high temperature pyrolysis
227
petroleum coke was graphitized in CO2 at higher temperatures (above 800 oC), which resulted
228
in a well-structured char and the decrease of active site of the sample 11, 49. Consequently, the
229
rate of petroleum coke gasification gradually decreased.
230
The average gasification rate R0.5 ( R0.5 = 0.5 / t 0.5 ) was used to indicate gasification reactivity
231
of individual samples due to the variations of their properties, where t0.5 denotes the duration
232
of the half conversion
233
order of gum wood (0.38 /min) > Australian coal (0.21 /min) > petroleum coke (0.04 /min).
234
Normally, gasification reactivity is largely influenced by both specific surface area (SBET) and
235
crystalline structure of char
236
possesses a higher gasification reactivity
237
proportional to the ratio of stacking height of the carbon crystal to interlayer spacing (Lc/d002)
238
value
239
reactivity
240
were identified as important parameters for the evaluation of active sites of char, the higher
241
the values of Ctot and Cstr, the greater the gasification reactivity. In order to confirm the order
242
of reactivity, these parameters were measured and listed in Table 2. Results showed that the
243
relationship between CO2 gasification reactivity with BET surface area, crystalline structure
244
and CO2 chemisorbed volumes followed the same trend as the average gasification rate,
245
which proved that these parameters could be used for both quantitative and qualitative
246
evaluation of average gasification rate.
11
. Another research also showed that
50
. The gasification reactivity of the three samples was ranked in the
2, 11, 12
. In general, char with a larger BET specific surface area 51
while gasification reactivity is inversely
12
. The number of active sites of char also shows strong correlation with gasification 32, 33, 52
. Therefore, total chemisorbed (Ctot) and strong chemisorbed (Cstr) volumes
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Table 2: BET surface area, crystalline structure and CO2 chemisorption features of samples.
Sample
SBET (m2/g)
Lc/d002
Ctot (mg/g)
Cstr (mg/g)
Cwea (mg/g)
Gum wood
6.44
2.18
2.08
1.65
0.43
Australian coal
2.29
2.73
0.75
0.54
0.21
Petroleum coke
0.49
5.23
0.59
0.44
0.15
248
Therefore, it can be concluded that apart from operating temperature, the gasification
249
reactivity was highly influenced by four major factors, i.e. specific pore surface area, the
250
number of active sites, crystalline structure and AAEM content of the char. The graphitization
251
of petroleum coke that occurred in the presence of CO2 and at higher temperatures was
252
believed to be the main reason for the gradual decrement of gasification rate. Volatile matter,
253
oxygen content and the composition of mineral matters also played a key role in the overall
254
reactivity. Hence, it is normal that CO2 gasification rate of petroleum coke was significantly
255
lower than those of gum wood and Australian coal. However, thermal processing petroleum
256
coke at temperatures higher than 1200 ºC and for a longer period of time could make a near
257
complete conversion become possible.
258
Although high temperature and longer processing time favour high conversion efficiency for
259
petroleum coke, this approach is not always practical. Since AAEMs have catalytic effects for
260
gasification process, AAEMs in carbonaceous materials, such as coal and biomass, could
261
potentially catalyse CO2 gasification of petroleum coke. Gasification rate of petroleum coke at
262
relatively low temperatures could therefore be enhanced via co-processing with coal/biomass.
263
3.3 Combustion and gasification under non-isothermal conditions
264
The TGA profiles of petroleum coke, Australian coal and gum wood samples under air and
265
CO2 atmospheres are presented in Fig. 4. Gum wood were the most reactive sample in both
266
combustion and CO2 gasification. As seen in Fig. 4, the first and second main mass losses
267
occurred at temperature ranges of 200 - 400 ºC and 400 - 1000 ºC, respectively. The ACS Paragon Plus Environment
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temperature of the first stage (devolatilization) was found to be narrow as the decomposition
269
of hydrocarbons occurred rapidly, which could complete within 20 - 30 ms. The maximum
270
mass loss rate of the devolatilization stage in both cases was similar (about 10% deviation),
271
which suggested that oxidizing agent did not have significant impacts on devolatilization. The
272
second stage (char gasification) took place at a wide temperature range due to the low
273
reactivity of char and the heterogeneous nature of the reaction. Since air is a much stronger
274
oxidizing agent than CO2, consequently, the second maximum mass loss in air occurred at a
275
relatively low temperature (480 ºC) and a higher rate (6.3 wt%/min) whereas a higher
276
temperature (965 ºC) and a lower rate (2.7 wt%/min) were observed in CO2 atmosphere.
1200
100
(a)
PC (CO2)
AC (CO2)
800 60 600 40 400
GW (CO2)
20
AC (Air)
Temperature (oC)
1000
80
Mass (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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GW (Air)
0
0 0
277 278
10
20
30
40
50
60
Time (min)
(b)
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16 PC (Air)
- DTG (%/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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12
AC (Air)
8
PC (CO2) AC (CO2) GW (CO2)
4
0 0
200
400
800
1000
1200
Temperature (oC)
279 280 281
600
Figure 4: Non-isothermal combustion and gasification of individual samples under air and CO2 atmosphere.
282
3.4 Combustion characteristics
283
Table 3 summarises the combustion characteristics of individual samples. It is obvious that
284
gum wood showed the lowest ignition temperature (282 ºC), which was mainly attributed to
285
its high volatile content. In contrast, petroleum coke had the highest ignition temperature (494
286
ºC) followed by coal (444 ºC). Under the same experimental conditions, gum wood was easy
287
to be ignited while the ignition of petroleum coke and Australian coal was much more
288
difficult compared with gum wood.
289
Table 3: Combustion characteristics of the samples. DTGmax
tmax
Di x10 -2 Sc x10 -7
Sample
Tig (ºC)
Tp (ºC)
PC
494
556
15.7
641
25.3
2.8
7.1
AC
444
533
10.9
633
24.2
2.3
4.8
GW
282
329/474
16.9/6.2
540
14.0
10.0
33.0
(wt%/min)
Tb (ºC)
(min)
290
In Table 3 it can be seen that gum wood had two peak temperatures (Tp), representing the two
291
stages of mass losses. In contrast, only one peak temperature was noticed for petroleum coke
292
and Australian coal. Additionally, the peak temperature for gum wood was lower than the ACS Paragon Plus Environment
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other two samples. Burnout temperature (Tb) of all the samples followed similar trend as seen
294
in Table 3. These characteristics indicated that gum wood had the highest reactivity whereas
295
petroleum coke demonstrated the lowest reactivity. However, the differences of Tp and Tb
296
between petroleum coke and Australian coal were insignificant (23 ºC and 8 ºC, respectively),
297
which suggested that the decomposition and combustion of these samples occurred at a
298
similar temperature range.
299
Table 3 showed that the values of ignition index and combustion index of the tested samples
300
followed almost the same pattern as the ignition temperatures. Therefore, gum wood is
301
considered a good fuel due to its low ignition temperature (282 ºC), short ignition time (14
302
min), low burnout temperature (540 ºC), and a high maximum burning rate (16.9 wt%/min).
303
Despite Australian coal had lower ignition temperature (444 oC) and shorter tmax (20 min), its
304
ignition index value was only 17% smaller than those of petroleum coke which was due to the
305
occurrence of higher value of maximum burning rate (15.7 wt%/min) in petroleum coke.
306
These findings suggested that besides ignition temperature, other combustion parameters,
307
such as maximum burning rate, also had significant impacts on fuel performance.
308
3.5 CO2 gasification characteristics of the samples
309
Table 4 shows the gasification characteristics of individual samples under CO2 atmosphere.
310
The initial devolatilization temperature of gum wood (305 ºC) was clearly the lowest,
311
followed by that of Australian coal (422 ºC). Two stages of mass losses took place resulting in
312
two maximum mass loss rates as illustrated in Fig. 4b. The first mass loss can be attributed to
313
the release of volatiles whereas char gasification contributed to the second mass loss. It is
314
important to note that unlike combustion process, the heterogeneous reaction between coal-
315
derived char and CO2 was much slower. This phenomenon can be explained by Boudouard
316
reaction (C(s) + CO2 → 2CO). Normally, at atmospheric condition, the Boudouard reaction is
317
thermodynamically favourable at temperatures above 900 ºC whilst the combustion of char ACS Paragon Plus Environment
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318
occurrs at significantly lower temperatures (below 630 ºC). This is because air is a much
319
stronger oxidizing agent compared with CO2. On the other hand, for petroleum coke its
320
devolatilization started at a much higher temperature (Ti=1110 ºC) due to its low volatile
321
content and high heavy aromatic-to-aliphatic ratios (aromatic carbon atoms are less reactive
322
than aliphatic atoms
323
delayed and occurred at higher temperatures. In addition, more ordered carbon crystalline
324
structure existed in petroleum coke also contributed to its low reactivity
325
from Table 4 that a higher maximum mass loss rate and corresponding temperature followed
326
the order of gum wood > Australian coal > petroleum coke. Generally, the reactivity of the
327
samples is directly proportional to the rate of maximum mass loss and inversely proportional
328
to the maximum temperature 1, 37, which is also dependent on the type of oxidizing agent used.
329
Hence, petroleum coke required a much higher temperature (1142 ºC) and a longer time (54.6
330
min) its gasification reaction under CO2 atmosphere to complete. In order to raise the
331
reactivity of petroleum coke, its co-processing with reactive fuels, such as gum wood, could
332
be one of the viable options in utility boilers since co-processing is normally associated with
333
interactions (synergistic effects) between fuels.
334
335
45
). Consequently, devolatilization of petroleum coke was considerably
11, 12
. It can be seen
Table 4: Gasification characteristics of the samples. DTGmax
Sample
Ti
Tp (ºC)
tmax (min)
PC
1110
1142
54.6
5.7
AC
422
460/1123
20.5/53.7
6.0
GW
305
361/961
15.6/45.5
14.4
(wt%/min)
Ti = Initial devolatilization temperature
336
3.6 Characteristics of co-processing
337
Fig. 5 shows the combustion profile of petroleum coke, Australian coal, gum wood and their
338
blends. It can be noticed in Fig. 5a that the peak temperature of petroleum coke/coal blend
339
(1:1) decreased slightly (by 6 ºC) compared with that of petroleum coke. In addition, 13% ACS Paragon Plus Environment
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reduction of maximum mass loss rate of the blend was also witnessed as the reactivity of coal
341
found to be lower compared with that of petroleum coke. As shown in Fig. 5b, gum wood had
342
a significantly lower ignition temperature (Table 2) so that it started to burn at a temperature
343
lower than that of petroleum coke which resulted in the first mass loss stage of petroleum
344
coke/gum wood blend (1:1). Therefore, the decomposition of petroleum coke started when
345
devolatilization of gum wood had completed. The first and the second peak temperatures of
346
the blend represented the first peak temperature of gum wood and petroleum coke,
347
respectively. It is also found that the maximum mass loss rate decreased by 54% in the
348
devolatilization stage due to a small amount of volatiles being added from gum wood.
349
Meanwhile, the increase of maximum mass loss rate by 100% in char gasification stage was
350
also observed, which was mainly caused by the increasing amount of char derived from
351
petroleum coke.
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(a)
20
PT for PC
- DTG (%/min)
15
PT for PC:AC
PT for AC 10
5
0 0
352 353
200
400
600
800
1000
1200
Temperature (oC)
(b) 20
PT for GW PT for PC 15 PT for PC:GW - DTG (%/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 20 of 29
1st PT for PC:GW 10
5
0 0
200
400
600
800
1000
1200
Temperature (oC)
354 355
Figure 5: Combustion profile of petroleum coke and its blends with (a) Australian coal and (b) gum wood.
356
Fig. 6 illustrates the CO2 gasification profiles of petroleum coke, coal, biomass and their
357
blends. In comparison with combustion process, similar thermal profiles were observed for
358
petroleum coke/Australian coal blend. However, the blend of petroleum coke/gum wood
359
exhibited a different trend than that of petroleum coke, where its second peak temperature
360
shifted to a lower temperature by 35 ºC. This suggested the presence of interactions during co-
361
gasification.
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(a)
8
PT for AC
- DTG (%/min)
6
PT for PC
1st PT for PC:AC
4
2
0 0
362 363
200
400
600
800
1000
1200
Temperature (oC)
(b) 20
PT for GW 16 - DTG (%/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1st PT for PC:GW 12 PT for PC 8 PT for PC:GW 4
0 0
200
400
600
800
1000
1200
Temperature (oC)
364 365 366 367
Figure 6: Gasification profiles of petroleum coke and its blends with (a) Australian coal and (b) gum wood.
368
3.7 Enhanced reactivity via co-processing
369
Interactions between fuels during co-processing are mainly attributed to the high volatile
370
content and AAEMs in biomass 15, 38, 53. These interactions could then be purposely utilized to
371
enhance the reactivity of petroleum coke, a fuel with low reactivity. In this study, an
372
Australian coal and gum wood were chosen to improve reactivity of petroleum coke by taking
373
advantages of the synergistic effects of co-processing.
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374
Figures 7 and 8 illustrate the curves of co-combustion and CO2 co-gasification processes,
375
respectively. Fig. 7a shows the deviations between experimental and calculated DTG curves
376
of petroleum coke/Australian coal blend in the temperature region of 350-650 ºC, which
377
indicated the existence of interactions during co-combustion. However, both experimental and
378
calculated DTG curves matched well at temperatures below 350 ºC and beyond 650 ºC, which
379
suggested that, interactions were minimum in this two temperature ranges, which were
380
dominated by coal devolatilization and char burnout respectively. For petroleum coke/gum
381
wood blend, a significant difference was observed in the temperature range of 400-700 ºC as
382
shown in Fig. 7b, which is a sign of strong interactions in this region. This deviation was due
383
to the presence of highly reactive gum wood-derived char and its high AAEMs’ catalytic
384
effects on gasification. In general, the experimental curves of both blends shifted to the low
385
temperature zone as demonstrated in Fig. 7. This shift depicted that the combustion of blend
386
samples occurred at a lower temperature, which indicated the enhanced reactivity due to
387
interactions between the two components in the blend. However, the deviation in petroleum
388
coke/gum wood blend was more significant and showed stronger interactions, which resulted
389
in the mass loss rate of experimental curve much higher than that of calculated one. These
390
behaviours also suggested that gasification reactivity was enhanced during co-processing.
391
Moreover, the interaction index (RMSII) of the blends was evaluated in order to determine
392
intensity of the interactions. The RMSII values of individual blends were calculated within
393
the previously mentioned temperature region where the deviation occurred. For petroleum
394
coke/gum wood blend, the RMSII value was 0.55, which was 22% higher that that of
395
petroleum coke/Australian coal blend (0.45). This indicated that the overall reactivity of
396
petroleum coke/gum wood was much higher than that of petroleum coke/Australian coal.
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(a)
16
AC:PC(1:1)-EXP
DTG (%/min)
12
AC:PC(1:1)-CALC 8
4
0 0
397 398
200
400
600
800
Temperature
1000
1200
(oC)
(b) 16
GW:PC(1:1)-EXP 12 DTG (%/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
GW:PC(1:1)-CALC 8
4
0 0
200
400
600 Temperature
399
800
1000
1200
(oC)
400 401
Figure 7: Experimental and calculated DTG curves in co-combustion (a) petroleum coke/Australian coal and (b) petroleum coke/gum wood.
402
Fig. 8 shows the correlation between experimental and calculated curves of the two blends
403
under CO2 atmosphere. In both blends, it can be seen that there was a good correlation up to
404
approximately 900 ºC. However, significant deviation was observed at the temperature range
405
between 1050 and 1200 ºC for petroleum coke/Australian coal blend (Fig. 8a), which suggest
406
the existence of synergistic effects.
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(a) 8
AC:PC(1:1)-CALC
DTG (%/min)
6
AC:PC(1:1)-EXP
4
2
0 0
407
200
400
600
800
1000
1200
Temperature (oC)
(b) 10 8
DTG (%/min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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GW:PC(1:1)-EXP
6
4
GW:PC(1:1)-CALC 2
0 0
200
400
600
800
1000
1200
Temperature (oC)
408 409 410
Figure 8: Experimental and calculated DTG curves in CO2 co-gasification (a) petroleum coke/Australian coal and (b) petroleum coke/gum wood.
411
Similarly, the curves of petroleum coke/gum wood (Fig. 8b) also experienced a notable
412
difference at higher temperatures (890 ºC to 1200 ºC). Moreover, the RMSII index indicated
413
that petroleum coke/gum wood blend had more significant interactions (2.82) compared with
414
that of petroleum coke/Australian coal (0.25). This suggested that the reactivity was higher in
415
the former one.
416
The aforementioned results demonstrated that a significant enhancement occurred in both
417
combustion and CO2 gasification when petroleum coke was blended with gum wood,
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418
especially at high temperatures. Between the two processes, CO2 co-gasification demonstrated
419
an enhanced performance by showing more significant interactions between individual fuels.
420
These interactions were attributed to chars derived from gum wood which were of high BET
421
surface area and have more active sites, lower crystalline index; and higher AAEMs (and
422
therefore stronger catalytic effect). As seen in Table 2 that BET surface area and crystalline
423
index value of gum wood char were 1.8 times larger and 58% lower than those of Australian
424
coal char, respectively. These properties subsequently contributed to the increased
425
gasification reactivity. Similarly, CO2 chemisorbed volumes (Ctot and Cstr) were found to be in
426
the range of 2.5 to 3.0 times higher than those of Australian coal, which indicated enhanced
427
gasification reactivity. In addition, the reactivity was enhanced due to the presence of high
428
amount AAEMs in ash originated from gum wood (as listed in Table 1) at high temperature 1,
429
38, 53
430
the CaO and K2O in gum wood were found to be 7.6 wt% and 4.5 wt%, respectively. These
431
two species were the main component for catalytic effect. In contrast, ash from Australian
432
coal and petroleum coke contained much lower amount of AAEMs (CaO, K2O and MgO)
433
than those of gum wood. In addition, alumina acted as a deactivator for catalytic effect
434
Since gum wood ash had the lowest percentage of Al2O3 (17.2 wt%), it experienced the least
435
difficulty to undergo the catalytic activities.
436
Therefore, this study confirmed that physiochemical properties of carbonaceous material i.e.,
437
active sites in char, AAEMs content etc., played a vital role in enhancing gasification
438
reactivity in CO2 atmosphere. Due to the significant differences in various carbonaceous
439
feedstock in terms of volatiles, fixed carbon and mineral content, the corresponding char
440
reactivity was influenced
441
CO2 atmosphere for chars derived from the blend of gum wood and petroleum coke was
442
observed, which was a result of the interactions between biomass and petroleum coke. It is
. The mechanism of these interactions were detailed in elsewhere 38. As seen in Table 1,
54
.
52
. In this study, significantly enhanced gasification reactivity in
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443
proved that gasification reactivity of petroleum coke can be enhanced via co-processing with
444
gum wood. This is a promising approach to enhance gasification performance of fuels of low
445
reactivity such as petroleum coke.
446
4 Conclusions
447
In this study, it is proved that synergistic interactions, which usually occur during the co-
448
processing of two fuels, could be utilized to enhance the combustion/gasification reactivity of
449
unreactive petroleum coke. However, in order to achieve this, the carbonaceous materials that
450
are to be co-processed with petroleum coke must have high volatile content, high AAEMs
451
content, and can form chars of large BET surface area, more active sites and low crystalline
452
index. This novel approach of purposely utilization of synergistic interactions to enhance
453
reactivity of unreactive fuels is of significant importance as it enables the large scale use of
454
poor quality fuels, such as petroleum coke and low rank coals.
455
Acknowledgement
456
Part of this work is sponsored by Ningbo Bureau of Science and Technology under its
457
Innovation Team Scheme (2012B82011). The University of Nottingham Ningbo China is
458
acknowledged for providing scholarships to the first author.
459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475
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